Printed circuit boards (PCBs) sometimes have a separated power or ground plane. Differential mode having been propagated along the microstrip line is divided into multiple modes when the differential mode arrives at the intersection of the signal line and the slit. They are differential mode that reflects backward and runs through, slot line mode and parallel plane mode. In order to evaluate excitation of undesirable electromagnetic modes, "transmission factor" is defined as a power ratio of excited mode to incident mode and is expressed in a matrix form like S matrix. The transmission factor is obtained by the electromagnetic calculation near the slit, since it is determined only by the structure of the intersection between the signal line and the slit.